In the prior art an optical fiber for continuously detecting a low temperature rapidly detects a low temperature of a chilled fluid with accuracy along in its longitudinal direction. Such an optical fiber apparatus is particularly useful in fuel storage facilities, for it is a noteworthy safe low temperature detecting means that will not cause an explosion due to its complete insulation against the environment.
A conventional method for detecting a low temperature with an optical fiber comprises detecting variation of attenuation through the optical fiber in proportion to temperature change by a so-called back scattering method which measures back scattering of light transmitted through the optical fiber. Such variation of attenuation corresponding to temperature change may be caused by change of difference between refractive indexes of a core and a cladding depending on temperature change or by microbending of the optical fiber resulting from shrinkage of a coating material of the optical fiber due to temperature change. In order to improve sensitivity of such a low temperature detecting means, it is necessary to amplify change of attenuation corresponding to temperature change within an intended temperature range. However, acceptable variation of attenuation is only about several tens dB/Km at most since a requisite strength of light to be transmitted has its own lower limit, which inevitably limits the detectable temperature range. Accordingly, it is difficult for the conventional optical fibers to detect the variation of attenuation over a wide temperature range, and a detecting system including the conventional optical fiber tends to malfunction.
A generally known optical fiber through which attenuation varies with temperature change includes a plastic cladding optical fiber (hereinafter referred to as "PCF"). PCF consists of a core made of silica glass and a cladding made of a silicone resin. Reference is made to FIG. 1 to explain functions of PCF. In FIG. 1, silica glass as a core shows a stable refractive index against temperature changes (Line A) whereas, the refractive index of the silicone resin varies with temperature (Line B). Therefore, the difference between refractive indexes of the core and the cladding is decreased or even becomes negative as the temperature of PCF becomes lower. Thus, the strength of light transmitted is reduced corresponding to the decrease of the difference between the refractive indexes which results from temperature lowering.
However, PCF unavoidably encounters a temperature at which PCF becomes incapable of transmitting light, namely, a temperature at which the refractive index of the core becomes smaller than that of the cladding so that light is not transmitted. In order to lower said temperature, a material having a low refractive index at room temperature, such as a silicone resin, should be used as a cladding material, but such an optical fiber is accompanied by an increased Rayleigh scattering and brings about an undesirable problem in that attenuation becomes too great.